7,391 research outputs found
Harmonic lattice behavior of two-dimensional colloidal crystals
Using positional data from video-microscopy and applying the equipartition
theorem for harmonic Hamiltonians, we determine the wave-vector-dependent
normal mode spring constants of a two-dimensional colloidal model crystal and
compare the measured band-structure to predictions of the harmonic lattice
theory. We find good agreement for both the transversal and the longitudinal
mode. For , the measured spring constants are consistent with the
elastic moduli of the crystal.Comment: 4 pages, 3 figures, submitte
"Poof! a'm heppily saving the Lord...": multimodality and evaluative discourses in male toilet graffiti at the University of the Western Cape
This paper explores the use of punctuation, capitalisation, linguistic forms and
images in the construction of evaluative discourses in male toilet graffiti at the
University of the Western Cape. Of particular interest is how male students use
these devises in the discursive construction of the appraisal resource of Attitude,
Graduation and Evaluation. Using over 150 tokens of graffiti, the paper uses a multimodal
approach employing notions of resemiotisation and remediation to show
how taboo language, font size, images and sketches are repurposed to aid the evaluation
of the 'self' and the 'other' in toilet graffiti. The paper shows that through utilising
multimodal texts, graffiti writers are able to reformulate and situate novel
meanings in contexts; and in terms of appraisal, the verbal and non-verbal semiotic
material are strategically combined to engender novel evaluations
Investigation of conduction band structure, electron scattering mechanisms and phase transitions in indium selenide by means of transport measurements under pressure
In this work we report on Hall effect, resistivity and thermopower
measurements in n-type indium selenide at room temperature under either
hydrostatic and quasi-hydrostatic pressure. Up to 40 kbar (= 4 GPa), the
decrease of carrier concentration as the pressure increases is explained
through the existence of a subsidiary minimum in the conduction band. This
minimum shifts towards lower energies under pressure, with a pressure
coefficient of about -105 meV/GPa, and its related impurity level traps
electrons as it reaches the band gap and approaches the Fermi level. The
pressure value at which the electron trapping starts is shown to depend on the
electron concentration at ambient pressure and the dimensionality of the
electron gas. At low pressures the electron mobility increases under pressure
for both 3D and 2D electrons, the increase rate being higher for 2D electrons,
which is shown to be coherent with previous scattering mechanisms models. The
phase transition from the semiconductor layered phase to the metallic sodium
cloride phase is observed as a drop in resistivity around 105 kbar, but above
40 kbar a sharp nonreversible increase of the carrier concentration is
observed, which is attributed to the formation of donor defects as precursors
of the phase transition.Comment: 18 pages, Latex, 10 postscript figure
Nanomaterial interactions with biomembranes: Bridging the gap between soft matter models and biological context
Synthetic polymers, nanoparticles, and carbon-based materials have great potential in applications including drug delivery, gene transfection, in vitro and in vivo imaging, and the alteration of biological function. Nature and humans use different design strategies to create nanomaterials: biological objects have emerged from billions of years of evolution and from adaptation to their environment resulting in high levels of structural complexity; in contrast, synthetic nanomaterials result from minimalistic but controlled design options limited by the authors' current understanding of the biological world. This conceptual mismatch makes it challenging to create synthetic nanomaterials that possess desired functions in biological media. In many biologically relevant applications, nanomaterials must enter the cell interior to perform their functions. An essential transport barrier is the cell-protecting plasma membrane and hence the understanding of its interaction with nanomaterials is a fundamental task in biotechnology. The authors present open questions in the field of nanomaterial interactions with biological membranes, including: how physical mechanisms and molecular forces acting at the nanoscale restrict or inspire design options; which levels of complexity to include next in computational and experimental models to describe how nanomaterials cross barriers via passive or active processes; and how the biological media and protein corona interfere with nanomaterial functionality. In this Perspective, the authors address these questions with the aim of offering guidelines for the development of next-generation nanomaterials that function in biological media
The Role of the Environment in Chaotic Quantum Dynamics
We study how the interaction with an external incoherent environment induces
a crossover from quantum to classical behavior for a particle whose classical
motion is chaotic. Posing the problem in the semiclassical regime, we find that
noise produced by the bath coupling rather than dissipation is primarily
responsible for the dephasing that results in the ``classicalization'' of the
particle. We find that the bath directly alters the phase space structures that
signal the onset of classical chaos. This dephasing is shown to have a
semiclassical interpretation: the noise renders the interfering paths
indistinguishable and therefore incoherent. The noise is also shown to
contribute to the quantum inhibition of mixing by creating new paths that
interfere coherently.Comment: 10 pages RevTex. Three figures in Postscript as a uuencoded
compressed tar file have been submitted as wel
Measurement of the Eta Production in Proton Proton Collisions with the COSY Time of Flight Spectrometer
The reaction pp -> pp eta was measured at excess energies of 15 and 41 MeV at
an external target of the Juelich Cooler Synchrotron COSY with the Time of
Flight Spectrometer. About 25000 events were measured for the excess energy of
15 MeV and about 8000 for 41 MeV. Both protons of the process pp eta were
detected with an acceptance of nearly 100% and the eta was reconstructed by the
missing mass technique. For both excess energies the angular distributions are
found to be nearly isotropic. In the invariant mass distributions strong
deviations from the pure phase space distributions are seen.Comment: 15 pages, 14 figures, 4 table
The polarizability model for ferroelectricity in perovskite oxides
This article reviews the polarizability model and its applications to
ferroelectric perovskite oxides. The motivation for the introduction of the
model is discussed and nonlinear oxygen ion polarizability effects and their
lattice dynamical implementation outlined. While a large part of this work is
dedicated to results obtained within the self-consistent-phonon approximation
(SPA), also nonlinear solutions of the model are handled which are of interest
to the physics of relaxor ferroelectrics, domain wall motions, incommensurate
phase transitions. The main emphasis is to compare the results of the model
with experimental data and to predict novel phenomena.Comment: 55 pages, 35 figure
Scattering in flatland: Efficient representations via wave atoms
This paper presents a numerical compression strategy for the boundary
integral equation of acoustic scattering in two dimensions. These equations
have oscillatory kernels that we represent in a basis of wave atoms, and
compress by thresholding the small coefficients to zero. This phenomenon was
perhaps first observed in 1993 by Bradie, Coifman, and Grossman, in the context
of local Fourier bases \cite{BCG}. Their results have since then been extended
in various ways. The purpose of this paper is to bridge a theoretical gap and
prove that a well-chosen fixed expansion, the nonstandard wave atom form,
provides a compression of the acoustic single and double layer potentials with
wave number as -by- matrices with
nonnegligible entries, with a constant that depends on the relative
accuracy \eps in an acceptable way. The argument assumes smooth, separated,
and not necessarily convex scatterers in two dimensions. The essential features
of wave atoms that enable to write this result as a theorem is a sharp
time-frequency localization that wavelet packets do not obey, and a parabolic
scaling wavelength (essential diameter). Numerical experiments
support the estimate and show that this wave atom representation may be of
interest for applications where the same scattering problem needs to be solved
for many boundary conditions, for example, the computation of radar cross
sections.Comment: 39 page
Ab initio optical properties of Si(100)
We compute the linear optical properties of different reconstructions of the
clean and hydrogenated Si(100) surface within DFT-LDA, using norm-conserving
pseudopotentials. The equilibrium atomic geometries of the surfaces, determined
from self-consistent total energy calculations within the Car-Parrinello
scheme, strongly influence Reflectance Anisotropy Spectra (RAS), showing
differences between the p(2x2) and c(4x2)reconstructions. The Differential
Reflectivity spectrum for the c(4x2) reconstruction shows a positive peak at
energies < 1 eV, in agreement with experimental results.Comment: fig. 2 correcte
Boundary-integral approach to the numerical solution of the Cauchy problem for the Laplace equation
We present a survey of a direct method of boundary integral equations for the numerical solution of the Cauchy problem for the Laplace equation in doubly connected domains. The domain of solution is located between two closed boundary surfaces (curves in the case of two-dimensional domains). This Cauchy problem is reduced to finding the values of a harmonic function and its normal derivative on one of the two closed parts of the boundary according to the information about these quantities on the other boundary surface. This is an ill-posed problem in which the presence of noise in the input data may completely destroy the procedure of finding the approximate solution. We describe and present the results for a procedure of regularization aimed at the stable determination of the required quantities based on the representation of the solution to the Cauchy problem in the form a single-layer potential. For given data, this representation yields a system of boundary integral equations with two unknown densities. We establish the existence and uniqueness of these densities and propose a method for the numerical discretization in two- and three-dimensional domains. We also consider the cases of simply connected domains of the solution and unbounded domains. Numerical examples are presented both for two- and three-dimensional domains. These numerical results demonstrate that the proposed method gives good accuracy with relatively small amount of computations
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